4sgb

Biological Background
Streptomyces griseus proteinase B (SGPB) is a bacterial serine endoproteinase with chymotrypsin-like specificity (see 5cha), isolated from Pronase, the commercial preparation of the extracellular fluid from S. griseus. The enzyme has 185 amino acids, with a molecular weight of 18,600. The sequence of SGPB has been aligned with α-chymotrypsin, and is similar in topology to other mammalian serine proteinases (Fujinaga et al., 1985).

Polypeptide chymotrypsin inhibitor-1 (PCI-1) is one of several proteinase inhibitors that have been isolated from Russet Burbank potato tubers (Pearce et al., 1982). The inhibitor is heat stable, with 51 amino acids in the form in this study, with a molecular weight of 5,600. There are 4 disulphide bridges in the structure.

PCI-1 and the large number of inhibitors like it, represent an interesting study, since their mechanism of action is not apparent by inspection of their sequence, or even their tertiary structure. They resemble standard protein substrates whose reactive site loops should undergo cleavage by the target enzyme.

Although they all have different tertiary structures, these inhibitors have some common features, such a wedge or pear shape, with the reactive site loop being the thin edge or narrow end of the molecule. Their reactive site loops which bind to the target enzyme all have similar conformations. Furthermore, in all but a few examples, the reactive site loops are constrained at both ends by disulphide bridges.

More information on the families of these inhibitors can be found in Laskowski and Qasim (2000).

Structure of PCI-1
PCI-1 is a disk-shaped molecule, with little regular secondary structure, but containing four disulphide bridges. One region of anti-parallel b-sheet with three strands opposite the reactive site is the most prominent piece of secondary structure. Aside from this there is a b-bulge (Ile23I, Cys24I, and Tyr51I), as well as a small helical feature involving Cys6 and Cys7.

The reactive site of PCI-1 extends from Ala35I to Pro42I. The side chain of Leu38I fits into the specificity pocket of the SGPB, and the peptide bond between Leu38I and Asn39I represents the scissile bond of a normal substrate. The bond is intact in the complex.

Reference
Refined structure of alpha-lytic protease at 1.7 A resolution. Analysis of hydrogen bonding and solvent structure. Fujinaga M, Delbaere LT, Brayer GD, James MN. J Mol Biol. 1985 Aug 5;184(3):479-502. PMID:3900416

Structure of the complex of Streptomyces griseus proteinase B and polypeptide chymotrypsin inhibitor-1 from Russet Burbank potato tubers at 2.1 A resolution. Greenblatt HM, Ryan CA, James MN, J Mol Biol. 1989 Jan 5;205(1):201-28. PMID:2494344

What can the structures of enzyme-inhibitor complexes tell us about the structures of enzyme substrate complexes? Laskowsk, M Jr., Qasim, MA. (2000). Biochim Biophys Acta 1477, pp. 324-337.

Isolation and characterization from potato tubers of two polypeptide inhibitors of serine proteinases. Pearce G, Sy L, Russell C, Ryan CA, Hass GM. Arch Biochem Biophys. 1982 Feb;213(2):456-62. PMID:6803670

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